The present invention relates to a base station for communicating with one or more mobile terminals of a wireless radio communication system, to a mobile terminal for communicating with a base station of a wireless radio communication system and a communication method for the communication between a base station and one or more mobile terminals of a wireless radio communication system. Particularly, the present invention relates to the power control of downlink information transmitted from a base station to one or more mobile stations in a short range wireless local area network.
In certain wireless local area communication systems as the proposed ETSI BRAN Hiperlan 2 system proposed for a short range wireless communication, as for example having a transmission range of 50 m to 100 m, a downlink slow or semistatic power control is used for the transmission of downlink information in order to reduce interference to neighbouring cells and systems. In the paper HL 13.5 ERI 2 a of May 12, 1999 for the planned ETSI BRAN standard, several proposals are disclosed for such a slow or a semistatic power control. In a wireless local area communication systems, as the proposed ETSI BRAN Hiperlan 2 system, user and control data are transmitted in so-called medium access control (MAC) frames. A MAC frame is a basic transmission frame and contains for example a broadcast control channel (BCCH), a frame control channel (FCCH), downlink data, uplink data and a random access channel (RACH). It is to be noted that the above specified communication system is a time division duplex (TDD) system, in which uplink and downlink transmission of user data is possible within one MAC frame. However, the present invention also relates to wireless radio communication systems in which the basic transmission frames or MAC frames only comprise either downlink or uplink data, as e. g. in a FDD system. Downlink transmission is the transmission of user or control data between a base station, also called access point or central control station, and one or more mobile terminals of the wireless communication system. Mobile terminals in the present context are all devices being dependent on or being controlled by a base station, e. g. in a Master-Slave relationship. The word xe2x80x9cmobilexe2x80x9d has been chosen to indicate that this kind of device may be freely moved within the transmission range of a respective base station. Uplink transmission is the transmission of user and control data from a mobile terminal to the corresponding base station. The communication system is a wireless radio communication system, in which signals are transmitted in the radio range, as for example the GHz range.
In the above-mentioned ETSI BRAN paper, several possibilities of a power control for the downlink transmission are described in order to avoid interference with other cells of the communication system or with other communication systems. One possibility is to assign downlink transmitter power independently to each mobile terminal communicating with the respective base station. This requires a complex design of the base station, since the transmitter power has to be changed several times during the downlink phase of one MAC frame duration. Additionally, the mobile terminals have to measure the received transmission power and communicate corresponding information to the base station. Since it is proposed to use the same downlink transmission power for all mobile terminals, the transmission power should be chosen so that all mobile terminals in one cell have sufficient reception.
In this case, one possible solution is to transmit the BCCH and the FCCH with a fixed power and to apply power control only to the rest of the downlink data transmitted in one MAC frame. Thereby, provisions have to be made in the base station in order to change the transmission power within one MAC frame and the automatic gain control in the receiving mobile terminals must be quite complex, since it has to be able to cope with the fast changes in the received transmission power between the BCCH/FCCH and the user data within a MAC frame. This proposal therefore leads to a high circuit complexity and a high power consumption on both sides, i.e. the base station and the mobile terminals, particularly since in all MAC frames the BCCH/FCCH and the user data are transmitted with different transmission powers.
A second proposed possibility is to use the same downlink transmission power in the entire MAC frames, i.e. power control is applied to the BCCH/FCCH as well as the downlink user data. Thereby, the mobile terminals within one cell communicating with a base station have to measure the received downlink transmission power and send corresponding information to the base station. On the basis of the received information the base station sets the downlink power control so that the mobile terminal having the worst reception still receives the downlink information with a sufficient quality. This possibility, however, has the disadvantage that the effective cell size is reduced to the actual active (communicating) mobile terminals. Since the effective cell size is limited, new mobile terminals can only gain access to the base station if they are within this limited coverage range.
The object of the present invention is therefore to provide a base station, a mobile terminal and a communication method of a wireless radio communication system of the above-mentioned type, which support a transmission power control of downlink information without limiting the transmission range only to currently active mobile terminals.
This object is achieved by a base station for communicating with one or more mobile terminals of a wireless radio communication system according to claim 1, with transmitting means for transmitting information in medium access control frames of said communication system, power control means for controlling the transmission power of the transmitting means so that the transmission power of at least a broadcast control channel and a frame control channel in a respective medium access control frame is increased after certain time intervals to an increased transmission power level as compared to the transmission power level of other medium access control frames (namely the downlink sections thereof) transmitted during the time intervals.
The above object is further achieved by a mobile terminal for communicating with a base station of a wireless radio communication system according to claim 12, with receiving means for receiving information in medium access control frames of said communication system, processing means for processing received information, adjusting means for adjusting the processing means to the transmission power of the received information, whereby the transmission power of at least a broadcast control channel and a frame control channel in a respective medium access control frame is increased after certain time intervals to an increased transmission power level as compared to the transmission power level of other medium access control frames (namely the downlink sections thereof) transmitted during said time intervals.
The above object is further achieved by a communication method for the communication between a base station and one or more mobile terminals of a wireless radio communication system according to claim 22, whereby information is transmitted and received in medium access control frames of the communication system, and whereby the transmission power of at least a broadcast control channel and a frame control channel in a respective medium access control frame is increased after certain time intervals to an increased transmission power level as compared to the transmission power level of other medium access control frames (namely the downlink sections thereof) transmitted during the time intervals.
It is to be noted that a medium access frame of the wireless radio communication system in the context of the present invention is a basic transmission frame comprising control and user data exchanged between a base station and a mobile terminal of the wireless radio communication system. Thereby, the increased power level transmission according to the present invention only concerns the transmission of downlink information, i. e. the transmission from a base station to one or more mobile terminals. Thus, a medium access control frame may either contain only downlink data, as e. g. in a FDD system, or contain downlink as well as uplink data, as e. g. in a TDD system. The present invention, however, only concerns the downlink data thereof.
According to the present invention, at least the BCCH and the FCCH of certain MAC frames (not all MAC frames) are transmitted with an increased power level so that new mobile terminals which are not covered with the actual effective cell size because of the normally performed slow or semistatic downlink transmission control are able to gain access to the base station by detecting the BCCH/FCCH transmitted with increased transmission power level. Thereby, either the BCCH and the FCCH of one MAC frame, one entire MAC frame (i.e. its downlink section including the BCCH, the FCCH and the downlink user data) or a couple of succeeding MAC frames (i.e. their downlink sections) can be transmitted with the increased transmission power level. The normal MAC frames transmitted during the certain time intervals between the increased power transmission are transmitted under a normal slow or a semistatic power control, so that the effective cell size is limited during these certain time intervals to the actually active mobile terminals. However, the transmission with the significantly increased transmission power level as compared to the normal transmission power level during the certain time intervals allows to connect new mobile terminals to gain access to the base station by listening to the BCCH/FCCH of the respective base station during a time period longer than the respective certain time interval. Thus, even mobile terminals outside the current effective coverage range of the base station can receive the essential BCCH/FCCH information and gain access to the base station even if the base station uses slow or semistatic downlink transmission power control. On the other hand, since the downlink transmission power control is performed during most of the time, interference to other cells and other systems is still avoided to a very large extent. Further, simple processing implementation is possible in the base station as well as in the mobile terminals.
Advantageously, the increased transmission power level of the base station according to the present invention is an absolut transmission power level e. g. a preset maximum power level of the communication system. Thereby, the increased transmission power level is always the same irrespective of the currently set transmission power level of the normal MAC frames transmitted during the certain time intervals.
Alternatively, the increased transmission power level is set relative to the power level of the normal medium access control frames transmitted during said certain time intervals. In this case, for example a certain transmission power level amount could be added to the currently set power level of the normal MAC frames.
In both cases, the increased power level (absolut or relative) may be varied in the base station according to system requirements. Thereby, this information has to be signalled to the connected mobile terminals.
Further advantageously, the certain time intervals are regular time intervals set by the power control means of the base station according to the present invention so that the transmission means transmits at least a broadcast control channel and a frame control channel of a respective medium access control frame regularly with said increased transmission power level after a fixed number of medium access control frames transmitted with a lower transmission power. The regular transmission of at least the BCCH and the FCCH of a MAC frame has the advantage that only simple processing is necessary. Particularly, mobile terminals trying to gain access to the base station only have to listen during a fixed time period being slightly longer than the regular time interval between two increased power transmissions to assure a guaranteed reception of the BCCH and FCCH. In this case, a mobile terminal only has to have knowledge about the length of the time intervals between the transmission with increased transmission power, i.e. the repetition rate.
Advantageously, the power control means of the base station sets the regular time intervals on the basis of a current medium access control frame number and a pre-set repetition rate. In this case, the mobile terminals which are already connected to the base station are able to synchronise in a simple and easy way to the transmission with the increased power level, which is important since the receiving processing of the mobile terminals has to be adapted to the received transmission power in order to enable a correct processing. Thereby, the transmitting means of the base station may transmit the repetition rate to the mobile terminals. The mobile terminals are then able to adapt their receiving processing to the reception of increased power levels by implementing the same scheduling algorithm as the base station.
Instead of transmitting the repetition rate to the mobile terminals a fixed repetition rate could be stored in the base station and the mobile stations, respectively. In another alternative, the base station may comprise means for providing information on the transmission time and/or the power level of a respective medium access control frame in which at least a BCCH and a FCCH are transmitted with said increased transmission power level, whereby the transmitting means of the base station transmits the information to one or more mobile terminals before the transmission of at least the BCCH and the FCCH with the increased transmission power level. By announcing the transmission with the increased transmission power level shortly before the actual transmission with the increased transmission power level the mobile terminals do not need to have any scheduling algorithm in order to be able to adopt to the reception of the increased transmission power levels, but may wait for the information from the base station announcing the power change and adopt the receiving processing correspondingly.
Alternatively to the transmission with the increased power level at regular intervals, the certain time intervals may be random time intervals set by the power control means of the base station so that the transmission means of the base station transmits at least a BCCH and a FCCH of a respective MAC frame after a randomly set number of MAC frames transmitted with the normal (lower) transmission power. The random time intervals allow a further decrease of the interference risk.
In this case, the base station according to the present invention advantageously comprises a random number generating means for generating a random number for each MAC frame to be transmitted, whereby the power control means sets the random time intervals on the basis of the random numbers, a respective current medium access control frame number and a repetition rate. Thereby, the random number generating means can for example be a linear feedback shift register providing pseudo random numbers. The use of random numbers, respective current medium access control frame numbers and a repetition rate enables the implementation of a simple and efficient scheduling algorithm, which may also be used in the mobile terminals to be able to adopt to the changing power levels. In this case, the base station of the present invention may transmit the repetition rate to the mobile terminals so that the repetition rate may be varied from time to time depending on system requirements. Alternatively, the repetition rate may be fixed and stored in both the base stations and the mobile terminals.
In order to avoid the necessity of implementing any scheduling algorithm in the mobile terminals, the base station may comprise means for providing information on the transmission time and/or the power level of a respective MAC frame in which at least a BCCH and a FCCH are transmitted with the increased transmission power level, whereby the transmitting means transmit the information to one or more mobile terminals of the communication system respectively before the transmission with the increased transmission power level.
As stated above, the adjusting means of the mobile terminal according to the present invention serves for adjusting the processing means of the mobile terminal to the transmission power of the received information, which is important since the significant change of the transmission power level according to the present invention may lead to problems in the receiving processing in the mobile terminal. Thus, the adjusting means of the mobile terminal has to be able to adjust the processing means in correspondence to the schedule of the transmission with an increased transmission power level by the base station of the present invention.
Advantageously, the adjusting means of the mobile terminal of the present invention is adapted to adjust the processing means to an increased transmission power level being an absolut transmission power level, e. g. a preset maximum power level of the communication system. In this case, the base station always uses the same transmission power level for the transmission of at least the BCCH and the FCCH of a MAC frame with the increased transmission power level. Hereby, no further processing is necessary to adopt the system to a varying enhanced power level. Alternatively, the adjusting means of the mobile terminal of the present invention may be adapted to adjust the processing means to an increased transmission power level being increased by a relative value in respect to the power level of MAC frames transmitted during the certain time intervals, i.e. transmitted with normal transmission power. Hereby, the increased transmission power level can be adopted to the power level of the normal MAC frames being power controlled in a normal way. In both cases, the absolutely or the relatively increased power level can be preset or can be signalled from the base station to the mobile terminal. The mobile terminal receiving the respective information then adapts its processing accordingly.
In case that the certain time intervals are regular time intervals, as explained above in relation to the base station, the adjusting means of the mobile terminal of the present invention regularly adjust the processing means to the increased transmission power level of at least the BCCH and the FCCH of a respective MAC frame after a fixed number of MAC frames transmitted with the normal (lower) transmission power. Hereby, no complicated processing implementation is necessary in a mobile terminal, since the mobile terminal only requires knowledge about the repetition rate or a fixed number of MAC frames transmitted with the normal transmission power as well as the time point of the transmission with the increased power level. Therefore, the mobile terminal advantageously comprises means for setting the regular time intervals on the basis of a current medium access control frame number and a repetition rate. These two pieces of information allow a simple and effective implementation of a scheduling algorithm so that the mobile terminal can adopt its receiving processing to the transmission level changes so that a proper processing of the received information is assured. Thereby, the repetition rate may by fixed and stored in a mobile terminal as well as in the base station, or the repetition rate is transmitted from the base station and received from the mobile terminal, so that the repetition rate may be varied from time to time depending on system requirements.
In case that the base station transmits at least a BCCH and FCCH with increased power level in random time intervals, the adjusting means of the mobile terminal according to the present invention adjusts said processing means to the increased transmission power level of at least the BCCH and the FCCH of a respective MAC frame after a randomly set number of MAC frames transmitted with the normal (lower) transmission power. Thereby, the mobile terminal may comprise setting means for setting the random time intervals on the basis of the random numbers, a respective current medium access control frame number and a repetition rate. In this way, the scheduling algorithm in the mobile terminal can be adopted in a simple and effective way to the scheduling algorithm of the base station so that a correct adjustment of the receiving processing to the changing power levels is assured. The mobile terminal thereby uses a random number and a current MAC frame number both received from the base station as well as a repetition rate for the scheduling algorithm. The repetition rate may either be received from the base station, so that it can be varied from time to time, or the repetition rate may be fixed and stored in the mobile terminal as well as in the base station.
In both above-mentioned cases, i.e. the regular and the random transmission of at least a BCCH and a FCCH with increased transmission power the implementation of a scheduling algorithm in a mobile terminal can be completely avoided when the base terminal transmits corresponding announcement information shortly before the transmission of at least a BCCH and a FCCH with the increased power level. In this case, the mobile terminal according to the present invention advantageously comprises detecting means for detecting said information on the transmission time and/or the power level of a respective MAC frame in which at least the BCCH and the FCCH are transmitted with the increased transmission power level respectively before the transmission of at least the BCCH and the FCCH with the increased transmission power level and for supplying the information to the adjusting means.